The mechanism by which virulent viruses such as mengovirus shut off host protein synthesis while their own protein synthesis is unaffected is not understood. This differential translation of host and viral messenger RNAs is not observed in vitro. Extracts (S-30 fractions) from uninfected and infected Ehrlich ascites tumor cells translate non viral and viral mRNAs equally well under different salt, magnesium ion and RNA concentrations. When viral RNA and non viral mRNAs are incubated simultaneously in a cell-free extract, the viral RNA appears to be translated preferentially over the non viral mRNA, suggesting that the viral RNA is a better competitor than non viral mRNAs for factors needed for translation. After infection of L-cells with mengovirus, several proteins associate with the 40S ribosomal subunit. The functional significance of this association in terms of the shut off of host protein synthesis is not understood and is being investigated. The proteins are fairly strongly bound to the subunits and are only released in the presence of 1M salt or ionic detergents. Mengovirus is translated into one or more large precursor proteins which are subsequently cleaved into functional proteins. Cleavage of the A precursor is inhibited by zinc ions. Using pulse-chase experiments it has shown that the zinc acts by altering the conformation of the growing A polypeptide, and not by inhibiting the enzymes involved in the cleavage of the A precursor. BIBLIOGRAPHIC REFERENCES: Abreu, S.L. and J. Lucas-Lenard. 1976. Cellular protein synthesis shutoff by Mengovirus: translation of non viral and viral mRNA's in extracts from uninfected and infected Ehrlich ascites tumor cells. J. Virol. 18:182-94. Nakai, K. and J. Lucas-Lenard. 1976. Processing of mengovirus precursor polypetides in the presence of zinc ions and sulfhydryl compounds. J. Virol. 18:918-25.